Germination of the grass weed Eleusine indica (L.) Gaertn. population as affected by temperature light and its response to glyphosate

Abstract

The grass weed Eleusine indica is listed as one of the most problematic weeds species in the world. It is characterized by vigorous growth and an extended root system that contributes to making this weed highly competitive. The discovery of glyphosateresistant (GR) biotypes across the world, which currently places E. indica in the top five in the list of GR weeds, has only exacerbated the problem status of the weed. Glyphosate is still regarded as the most important herbicide and has become the most historically successful herbicide throughout the world. Eleusine indica populations that proved to be resistant to glyphosate have been identified under various cropping situations and are now amongst the most widely distributed herbicide resistant species, causing great economic impact worldwide. This has triggered interest on the biology of this species and its response to glyphosate in order to improve its control. Several studies have been conducted on this species across the world but no studies have been conducted in South Africa. Therefore, there is a need for a better understanding of the biology of the weed and information on effective control of this cosmopolitan weed in South Africa as well as globally. The objectives of the present study were to determine the effects of temperature and light on seed germination of E. indicaas well as to investigate the response of E. indica to glyphosate. Growth chamber and glasshouse experiments were conducted on a population of Eleusine indica obtained from Hatfield experimental farm of the University of Pretoria (location: 25°45'7.08"S, 28°15'33.12"E). All experiments were conducted at phytotron facilities on the experimental farm. In the growth chamber, experiments were conducted to assess the germination behaviour of E. indica under different temperature and light regimes with different germination media. The lowest germination rate was recorded in H2O medium, under light/dark conditions (82.42%). Seeds germinated best in KNO3 medium regardless of alternating temperature regimes or light conditions. The highest final germination was recorded in KNO3 medium in the dark (99.17%). Germination of E. indica was high (>80%) at all three alternating day/night temperature regimes and light interactions. The response of E. indica to glyphosate was investigated in a growth chamber with a rapid in vitro experiment and in the glasshouse by means of a dose-response method. Two in vitro methods were tested with un-germinated and pre-germinated seeds placed on filter papers in Petri dishes to which solutions with glyphosate concentrations of 0 and 900 g a.e. ha-1 were added. After seven days, root and shoot lengths of the seedlings were measured. The shoot and root development of both the un-germinated and pregerminated seeds were significantly reduced by glyphosate. In the glasshouse, a doseresponse experiment was conducted to determine the response of E. indica to a range of glyphosate dosages. Glyphosate dosage rates were 0, 0.25x, 0.5x, 1x, 2x and 4x the registered label recommended rate of Roundup Turbo® (1x = 900 g a.e. ha-1). Biomass was measured 28 days after treatment. All glyphosate doses examined in the doseresponse experiment significantly reduced the biomass of E. indica. The findings of both these experiments reveal that the E. indica population examined in this study is sensitive to glyphosate. Finally, the effect of growth stage on the response of E. indica to glyphosate was investigated in the glasshouse. Plants were treated with glyphosate at the field recommended dose of Roundup Turbo®, 900 g a.e. ha-1 at four different growth stages: 21, 35, 49, and 63 days after emergence (DAE). Control treatments were maintained for comparison for each growth stage. Biomass was measured 28 days after treatment. While plant biomass was significantly reduced by glyphosate, older plants (63 DAE) were visually less affected by glyphosate. Despite glyphosate treatment, these plants continued to grow and produce seeds. Therefore, growth stage had an effect on the response of E. indica to glyphosate. The results of this study demonstrate the importance of growth stage on glyphosate application even where plants are sensitive to the herbicide. Growers should therefore consider and be aware of the importance of timing glyphosate application. There is a need to intensify this study to examine populations from various geographic locations. Germination characteristics and weed response methods for weeds from different locations might differ as a result of different climatic conditions and weed control practices. This information is essential for improving methods currently available for control of this weed in the country, and perhaps even in other parts of the world.